The invention relates to apparatus for unstacking flat items, such as mail items that may be closed or open. This type of apparatus is used in particular in machines for automatically sorting mail items or the like.
U.S. Pat. No. 4,357,007 discloses an apparatus for unstacking flat items that includes a first suction nozzle and a second suction nozzle disposed on one side of an alignment plate, a perforated endless belt advancing continuously past the suction nozzles and past the free face of the first item in a stack of items, a passage forming an outlet between the alignment plate and the belt, and a first sensor and a second sensor that are disposed on the other side of the alignment plate. The second suction nozzle is spaced apart from the first suction nozzle downstream in the advance direction of the endless belt, and the second sensor is spaced apart from the first sensor downstream in the advance direction of the belt. The second sensor causes either the first suction nozzle or the second suction nozzle to operate depending respectively on whether the first sensor detects an item or does not detect an item.
That known apparatus makes it possible to single items from a stack of items and to distribute them one by one towards the outlet even when some of the items in a stack of items standing on their edges and abutting against an alignment plate are misaligned.
A problem encountered in apparatus for unstacking items is that two consecutive items in a stack of items to be singled are not always disposed with their leading edges in contact with the alignment plate, thereby giving rise to malfunctions in the unstacking apparatus. For example, when a first item in the stack has its leading edge set back from the alignment plate and beyond the trailing edge of a suction nozzle, and when a second item has its leading edge in contact with the alignment plate, the second item and not the first item is taken first by the suction nozzle when said nozzle is actuated and said second item is then conveyed towards the outlet by the endless belt before the first item. If the first item in the stack is not displaced towards the alignment plate, said first item will never be unstacked.
In the above-mentioned document, first control means are provided for selectively actuating the suction nozzles in response to the signals supplied by the sensors. The control means operate as follows. If, at time t=0, the sensors detect the absence of an item at the outlet, the control means actuate the first suction nozzle until the first sensor detects the presence of the article, thereby causing the first suction nozzle to be deactivated, and the second suction nozzle to be actuated. The item retained against the endless belt by the second suction nozzle is displaced towards the outlet until the second sensor detects the presence of the article, thereby causing the second suction nozzle to be deactivated.
That known apparatus suffers from drawbacks. Firstly, the amount of misalignment that is acceptable between the leading edges of the items is equal to the distance between the trailing edge of the first suction nozzle and the surface of the alignment plate. But the maximum distance between the trailing edge of the first suction nozzle and the first sensor is equal to the size of the smallest item in the stack. Since the sensors are disposed on a different side of the alignment plate from the suction nozzles, the maximum acceptable misalignment is less than the size of the smallest item in the stack. Secondly, under normal operating conditions, i.e. when no article in the stack is set back out of alignment, the control means actuate the first suction nozzle and then the second suction nozzle simultaneously. As a result, the rate of unstacking is lower than the rate which would be obtained if the control means were to activate a single suction nozzle. An item that is referred to below as being "backwardly misaligned" is an item that has its leading edge set back from the alignment plate beyond the trailing edge of the second suction nozzle. The term "realignment" designates the action of bringing the leading edge of a misaligned item forward against the alignment plate once again.